The process of fabricating a semiconductor device into a final electric component includes the step of fixedly mounting a semiconductor chip having, for example, a pn junction formed therein and having a certain function, onto a support which is called a "stem" made of either an electroconductive metal or a non-conductive or insulating ceramic; and to bond a lead wire to a metal electrode ohmicly provided at a predetermined location on the surface of the chip, for a subsequent electric connection between the metal electrode and terminals of an external electric or electronic equipment.
Such fabricating step, especially in mass production, requires that it be completed with precision and during a short period of time. To this end, there arises the necessity for a quick and precise location of the metal electrode, i.e., the position thereof at which a lead wire is to be bonded. The metal electrode is in ohmic contact with a surface of the semiconductor chip which is fixedly mounted on a support or "stem"; and there is the necessity for driving a bonding apparatus such as a "wire bonder" in conjunction with the detection of the image and location of the ohmic contact metal electrode.
Typical conventional wirebonding apparatus includes a means for holding a lead wire and means for maneuvering the lead wire by orthogonal movement of the holding means. Servomechanisms are provided which move the holding means and the wire in each of three dimensional directions according to electrical signals conveyed to the servomechanisms. In conventional practice, the bonding apparatus may be controlled by manual manipulations from an operator viewing the end of the connecting wire through a microscope or otherwise.
More recent bonding apparatus that is controlled by sensors and optical images and video signals has been hindered by the quality of the images and particularly when the bond is to be made at a surface which is transparent.
With respect to this locating and bonding operation, various techniques have been developed so far, depending on the purpose of use or the type of the device which employes the semiconductor chip.
In case of a semiconductor device comprised of a Group III-V compound semiconductor material such as GaP, and especially in the case where the semiconductor material is transparent to infrared or visible rays, as in case of a GaP LED (light-emitting diode) which features a small absorption by the semiconductor material of those light rays in the region of wavelengths of visible light and infrared rays, it has bee the usual practice in a mass production system to illuminate the surface of the semiconductor chip for the purpose of locating that particular metal electrode on the surface of the chip to which a lead wire is to be bonded.
Description of the detecting and bonding process will be made hereunder in further detail with respect to the instance of a GaP LED, as an example of a semiconductor chip.
A GaP LED is a semiconductor device which has a pn junction formed therein. It is operative so that by the application of a forward bias across an ohmic metal electrode contact formed on a p type semiconductor region and a metal electrode provided ohmmicly on an n type semiconductor region, to thereby cause a forward current to flow across the pn junction to emit light therethrough. A basic structure of a simplest ordinary GaP LED model is schematically illustrated in FIG. 1, which is a vertical sectional view.
In FIG. 1, a GaP semiconductor chip 10 is basically divided into two adjacent semiconductor regions; i.e., a p type region 31 and a n type region 32. It should be understood that the conductivity types of these two regions 31 and 32 may be reversed. Two metal electrodes 11 and 12 are provided ohmicly on these two regions 31 and 32, respectively. The top surface of the semiconductor chip 10 has a light-emitting area 14. The bottom surface of the chip 10 is ohmicly provided with a metal electrode 12 fixedly mounted on a support (not shown) via, for example, an electroconductive paint containing silver or via a fusible metal solder which melts at a low temperature.
After fixedly mounting the chip 10 onto a support, a lead wire is bonded to the metal electrode 11, which is ohmicly provided on top of the chip 10, by relying on the technique of thermal compression bonding or ultrasonic thermal compression bonding. In order to facilitate the locating operation for the metal electrode on the surface of the chip 10, this surface is illuminated.
The operation of locating a metal electrode by means of illumination has been conducted usually by relying on the below mentioned techniques. These conventional techniques will be briefly described by referring to FIGS. 2 which are schematic perspective views of an apparatus arrangement.
FIGS. 2(a) and (b) illustrate the technique using an oblique illumination. A beam of light from a light source 15 is caused to impinge obliquely onto a top surface 14 of the semiconductor chip 10 that is ohmicly contacted locally by a metal electrode 11 and which serves as the light-emitting area surface 14, excepting the ohmic-contact area.
In cases of FIGS. 2(a) and (b), the beam of reflecting light rays, which is incident to a detector 16, such as an image sensor, represents a portion of those light rays reflected in scattering fashion from a rough surface of the chip. In these two cases specular light rays, which are reflected on the smooth flat surface, are eliminated.
FIG. 2(c) shows an instance wherein the top surface 14 of the chip 10 is illuminated by an oblique beam of light rays emitted from a light source 15, and the specularly reflected light beam off the smooth flat surface enters into the detector 16. In this instance, the light rays scattered by the rough surface portions are eliminated so that this technique may be termed the specular illuminated technique.
FIG. 2(d) illustrates a modification of the oblique illumination techniques of FIGS. 2(a) and (b). A beam of light rays emitting from a light source 15 is specularly reflected by mirors 17 and 18 to illuminate the surface of the semiconductor chip.
FIG. 2(e) is a modification of the specular illumination technique. A beam of light rays emitting from a light source 5 is specularly reflected by a light beam splitter 19, such as a half-mirror, and this reflected beam of light is used to illuminate the smooth and flat surface of a semiconductor chip 10. And, the beam of light which is specularly reflected off this surface passes through a light beam splitter 19 to impinge onto the detector 16.
It should be understood here that, in order to detect the position of the metal electrode region in good contrast to the light-emitting region of the semiconductor chip surface, it is a requirement that the images of these two regions be detected jointly by the detector. However, as stated earlier herein, semiconductor material, such as GaP, which may constitute a semiconductor chips, is by nature transparent to visible and infrared rays. Therefore, in any kind of conventional illumination techniques, the beam of light rays reflected off a GaP semiconductor chip surface and impinging onto the detector, consists of a component of light rays reflected from the top surface of the semiconductor chip, and a component of light rays which pass from the light-emitting area through the semiconductor material and are reflected at the metal electrode 12 or other metal layer provided at the bottom surface of this chip and emit again through the top surface of the chip.
As such, the metal electrode 11 which is ohmicly provided on top of the semiconductor chip, and the metal electrode 12 provided at the bottom thereof will be detected by the detector in substantially the same position or appearance. Thus, the detector is unable to sufficiently distinguish the metal electrode region 11 provided ohmicly on the top surface of the semiconductor chip from the region of the light-emitting area of the chip surface.
For the reason stated above, the conventional illumination techniques have the inconveniences and drawbacks such that a precise detection of the location of the metal electrode ohmicly provided on top of a semiconductor chip requires a lengthy time, and/or that the circuit arrangement for analyzing the image detected, even by a highly sophisticated detector, becomes complicated. Also the operation of the leadwire bonder, which operates in conjunction with the detector, becomes erroneous.
It is, therefore an object of the present invention to provide a wire bonder with an improvement in the illumination method which eliminates such inconvenience and drawbacks of the conventional illumination techniques as stated above.
Another object of the present invention is to provide a wire bonder with an illumination method of a type, which enhances the contrast between the image of light rays reflected from the surface of a semiconductor material and the image of light rays reflected from a metal electrode provided on the top surface of said semiconductor material by drastically varying the intensities of these two kinds of light rays.
Still another object of the present invention is to provide an illumination method and system of the type as described above, which allows the detection of the image as well as the location of the metal electrode provided ohmicly on the top surface of a semiconductor chip to be carried out by a relatively simplified optical or analyzing system, and to thereby quicken the precise bonding operation of the leadwire bonder in conjunction with the detector.
A yet further object of the present invention is to provide an illumination method and system of a type similar to that described above, which performs the detection of the image and position of the metal electrode in good contrast to the image and location of the semiconductor material regions of the chip by causing when, a beam of light illuminates the chip surface, a maximum polarization of light transmitted through the semiconductor material regions. This is done by casting the illuminating beam of light at Brewster's angle as defined between the axis of the beam of light and a line normal to the chip surface, and also by producing a maximum difference in value between the intensity of the light reflected off the metal electrode and the intensity of the light reflected off the semiconductor material by using a polarizer.
Although it is known that contrast can be enhanced between a mark on the surface of a relative transparent material by means of projecting and reflecting light at Brewster's angle as shown by U.S. Pat. No. 3,699,350 Holdaway, this advantage has not been previously conceived or taught in connection with a wirebonding operation and apparatus. The problem of obtaining a contrasting image with a physical object on the surface of a transparent material is involved. The reference patent teaches reflecting a beam of light on the surface of a polyster, glass, or other material having an index of refraction for the wavelength of radiant energy being used, and reflecting the light at an angle of incidence equal to Brewster's angle. The back of the substrate is coated with a light absorbent material such as iron oxide or flat back paint. By this means a mark or indicia is made to contrast and provide an electric signal on a photo detector. There is no teaching or understanding in the patent that a contrasting image of a physical object on the surface of a transparent substrate can be precisely contrasted on the surface of a semiconductor chip. The patent teaches that a gross indication can be achieved of the presence of a mark on the surface of the transparent substrate, if the function of preventing light from being internally reflected in the substrate and back to the photo pickup can be accomplished by several expedients, such as making the back surface of this substrate irregular or a darker absorbent color. The light absorbing layer on the opposite side of the substrate from the incidental light beam is taught to be very important in the patent.
U.S. Pat. No. 3,567,309 Jasgur teaches the glare can be reduced on the background of an object being viewed if the incidental light is polarized and a polarizing means is provided in the reflected light beam at a different angle of polarization with respect to the direction of the incident light.
It is an object of this invention to combine these prior art teachings in a new system for the automatic bonding of lead wires on a semiconductor chip by a bonding machine.